I just did this on my 64 Valiant wagon, and I too was tempted to keep the old races in place, since it's probably the most time consuming part of the job for me, but I was taught just what Rusty said. The old races have been worn in with the old bearings and now don't match the new bearings perfectly, so I would replace them. And It's definitely not a must but I like having the tool that makes it easiest, and for only about 30 or 40 bucks you can get a race installer set online, then you just tap the races in place, instead of needing a press. Also, I thought to myself when installing the spindle nut, what's the best way to achieve the right torque? These bearings need some preload right? Is there a torque spec? My method has just been to spin the hub around while tightening it, feel for some resistance but not too much, and install the lock and cotter pin. I haven't had issues doing it this way, but I'm always interested in knowing the correct way, or even just a better way.